The present invention relates to a method of increasing the tenacity and Young's modulus of elasticity of infusible cured phenolic resin fibers and to the improved fibers produced thereby.
Phenolic resins are too well-known in the art to require more than a very brief description here. Extensive discussions of phenolic resins may be found, for example, in A. A. K. Whitehouse et al, Phenolic Resins, American Elsevier Publ. Co., Inc., New York (1968), and Gould, Phenolic Resins, Reinhold Publ. Corp., New York (1959).
Phenolic resins are produced by the condensation of a phenol and an aldehyde. The phenol employed is most commonly phenol itself, but any of a wide variety of phenols as well as mixtures thereof may be used, such as phenol which is substituted in the ortho, meta, and/or para position, provided that sufficient ortho and para positions are unsubstituted to permit condensation and cross-linking. Similarly, various aldehydes have been employed, formaldehyde being by far the most commonly used. Accordingly, many different varieties of phenolic resins are commercially available.
Phenolic resins are generally classified as either resoles or novolacs. Resoles are ordinarily prepared by carrying out the condensation with a molar excess of the aldehyde and in the presence of an alkaline catalyst. Resoles are characterized by the presence therein of methylol groups, which render it possible to effect curing and cross-linking via methylene linkages by heat alone. Novolacs are usually prepared by employing an acid catalyst and a slight molar excess of the phenol. Novolacs are characterized by the absence of methylol groups, and accordingly, they cannot be cured and cross-linked by heat alone, additionally requiring the presence of a source of methylene groups and preferably a suitable catalyst.
Infusible cured phenolic resin fibers are a comparatively recent development in the history of phenolic resins. They are ordinarily produced by fiberizing a melt of a phenolic resin, as by melt spinning or by blowing (i.e., allowing a thin stream of the melt to fall into the path of a blast of a gas such as air which fiberizes the stream), to obtain fusible uncured phenolic resin fibers which are subsequently treated to cure, or cross-link, the resin at least to the point of infusibility. When the phenolic resin selected is a resole, such curing is effected merely by heating. When the phenolic resin selected is a novolac, curing is effected by heating in the presence of a source of methylene groups such as hexamethylenetetramine, paraformaldehyde or formaldehyde, and preferably also in the presence of an acidic or basic catalyst, hexamethylenetetramine being rather unique in being able to serve as both a methylene group source and a basic catalyst. A particularly desirable method for the preparation of infusible cured novolac fibers is described in U.S. patent application Ser. No. 710,292, filed Mar. 4, 1968 by James Economy et al, now U.S. Pat. No. 3,650,102, which is commonly assigned with the present application, and the disclosure of which is incorporated herein by reference. Fibers may also be prepared from mixtures of resoles and novolacs in any desired proportions, the curing conditions being selected with regard to the proportions. Additives and modifiers, either reactive or non-reactive, may be incorporated in the phenolic resin to alter its fiberization characteristics and/or the properties of the fibers.
Infusible cured phenolic resin fibers have a number of highly desirable properties which render them of value in numerous applications. Perhaps their most important virtue is their outstanding flame resistance. When subjected to a flame, the fibers, being infusible, do not melt, but rather char to produce carbon fibers which continue to retain the shape and approximate dimensions of the original fibers and which continue to afford extremely effective protection from flames. Accordingly, the fibers are of potentially great utility in the fabrication of flame protective clothing, as well as drapes, carpeting, upholstery and the like which are especially suited to use in areas where fire constitutes a particular hazard. Such fibers also provide very effective thermal and acoustical insulation, and again, they are particularly useful in these applications in areas where fire is a hazard.
Infusible cured phenolic resin fibers produced as described above are somewhat susceptible to oxidation, particularly at elevated temperatures. Just after curing, they are generally quite intensely colored, the hue ranging from fairly deep pink to red, sometimes with a somewhat orange cast; and upon standing, particularly if exposed to light and air, the coloration increases considerably in intensity, becoming deep orange, orange-red, or brownish-red; that is, the fibers possess rather poor colorfastness. It has recently been discovered that infusible cured phenolic resin fibers which are white and which have markedly improved colorfastness and oxidation resistance may be produced by blocking at least about 50%, and preferably at least about 90%, of the phenolic hydroxyl groups of the cured resin in the fibers by etherification or, preferably, esterification. This blocking of the phenolic hydroxyl groups has little or no effect upon the tenacity or Young's modulus of elasticity of the fibers. Infusible cured phenolic resin fibers wherein the phenolic hydroxyl groups of the cured resin are blocked and methods for the production thereof constitute the subject matter of U.S. patent application Ser. No. 130,017, filed Mar. 31, 1971 by James Economy et al, now U.S. Pat. No. 3,716,521, entitled ETHERIFIED OR ESTERIFIED PHENOLIC RESIN FIBERS AND PRODUCTION THEREOF which is commonly assigned with the present application and the disclosure of which is incorporated herein by reference. Blocking is readily carried out by reacting the infusible cured phenolic resin fibers with any of a wide variety of suitable esterification or etherification reagents whereby the hydrogen atoms of the phenolic hydroxyl groups are replaced and the phenolic hydroxyl groups are blocked by esterification or etherification. Suitable reagents include anhydrides of carboxylic acids, acylation with anhydrides of lower alkanoic acids being preferred, especially acetylation with acetic anhydride. Other suitable reagents include, for example, acid halides such as acetyl chloride, diethylsulfate, and dimethylsulfate.
Notwithstanding their desirable attributes, the utility of blocked and unblocked infusible cured phenolic resin fibers has heretofore been somewhat limited by their relatively poor mechanical properties, in particular, their relatively low tenacity and, to a lesser extent, their relatively low Young's modulus of elasticity. Such fibers typically have a tenacity in the range from about 1 to about 2 g./den., thus being strong enough to be suitable for certain applications but somewhat too weak for certain other applications. For example, fabrics produced from such fibers tend to have relatively poor strength and wear characteristics due to the relatively low tenacity of the fibers. Accordingly, infusible cured phenolic resin fibers having a somewhat higher tenacity would be highly desirable, from the standpoint of broadening the range of end use applications and of producing stronger fabrics capable of better wear performance. An increased Young's modulus of elasticity would also be beneficial in these respects.